Add graphviz install warning if binaries not found

[csala]

Resolve #182

[codecov-commenter]

Codecov Report

Merging #189 into master will increase coverage by 0.50%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
+ Coverage   77.87%   78.37%   +0.50%     
==========================================
  Files          22       22              
  Lines        1541     1683     +142     
==========================================
+ Hits         1200     1319     +119     
- Misses        341      364      +23     

Impacted Files	Coverage Δ
sdv/metadata/visualization.py	86.27% <16.66%> (-9.29%)	⬇️
sdv/metadata/errors.py	100.00% <0.00%> (ø)
sdv/metadata/table.py	84.49% <0.00%> (+2.22%)	⬆️
sdv/tabular/base.py	67.90% <0.00%> (+3.49%)	⬆️

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